Richard Saferstein (Former Chief Forensic Scientist) gives expert video advice on: What is forensic science?; What is forensic 'physical evidence'?; What is forensic 'identification' of physical evidence? and more...
What is forensic science?
Forensic science is the application of science to law, but in practice it is applied science to criminal law, because in many instances where crimes are committed, they require the skills of individuals that have a background in science or medicine. For example, an odontologist or dentist applies their skill of identifying individuals through their teeth; an anthropologist can apply their skills in terms of identifying bones for identification purposes. There are psychiatrists, there are psychologists, there are pathologists who are called upon to examine bodies and to carry out autopsies. There are toxicologists who examine human remains for the presence of poisons and drugs. There are individuals, such as myself, who have a background in chemistry. I was educated as a chemist, and I apply chemistry and biology to the law, and more specifically to criminal law. People who use chemistry and biology are typically employed in the crime lab.
What is forensic 'physical evidence'?
Physical evidence is any object that could link a suspect to a crime scene, a victim to a crime scene, or it can tell us something about whether a crime has taken place or not. Now, what is an object? That in itself is pretty interesting. An object can be anything such as a gun or it can be a bullet, but it also can be something as small as a human cell, because with current technology we can extract information from a single cell that can tell us something about the DNA type of an individual that was present at the crime scene. So I use the term 'object' very loosely. It can be anything from the large to the infinitely small.
What is forensic 'identification' of physical evidence?
Identification is what it is. What is a powder? Is it a drug that contains heroin, or is it a drug that contains cocaine? The ultimate result of the identification is the exclusion of all other substances but one. When a forensic scientist goes to court with the result of an analysis of a white powder and reports to the court, for example, that that white powder contains heroin, that forensic examiner will be as certain as to his or her findings and to the identity of his or her findings as modern technology will allow one to be. It must be a near absolute identification and exclude all other substances from consideration.
What is a forensic 'comparison analysis' of physical evidence?
Comparison is quite different than the "what is it" identification. The purpose of the comparison is to make a determination as to the common origin of items of evidence. Did the hair in the crime scene come from the head of a particular suspect? Did a fingerprint found at a crime scene come off the fingers of a certain person who was suspected of being the perpetrator? We undertake comparisons for the purposes of determining common origin.
What are forensic 'individualistic characteristics' of evidence?
Let me give you two examples. The first is the fingerprint. There are what we call minutiae or fingerprint ridge characteristics that a fingerprint specialist will look for and will match up between two fingerprints that are being compared. If the position and the type of ridge characteristics are identical and there are enough of them, the fingerprint specialist can then come to a conclusion that the two fingerprints came from the same individual. The other area that I would like to refer to would be DNA typing. We look for certain DNA types when we collectively examine an item of physical evidence for its' DNA character and, if there are enough of these different types of DNA that exist, we can then make the determination that the DNA emanated from only one individual.
What are forensic 'class characteristics' of evidence?
When we are dealing with class items of evidence, that is, items where we have made a comparison but we can only limit the comparison to a small number of individuals or a small group and not to one single person, and that may be very frustrating to a prosecutor or to a jury to hear. But, the point of fact is that often physical evidence cannot be linked to only one person. It is important to understand that kind of evidence, and that kind of class of evidence is very important in building a case. It will collaborate other types of police investigative evidence that may be created during the investigation; confessions, eyewitness testimony and the like.
How do forensic scientists compare class characteristics of evidence?
If, for example, we're comparing hairs we will look at the length of the hair, the diameter of the hair, and the color of the hair. All of this would be done under the comparison microscope. It is the same with fibers - if we're comparing fibers we're going to look at the color, were going to look at the diameter, we are going to look at their morphological appearance under the microscope and we're also going to examine that fiber for its chemical identity. It really depends what we're looking at, and there are so many different items or potentially items with physical evidence that could be collected at a crime scene. Each one must be examined individually and by a specialist who must pick out two of the appropriate characteristics that will ultimately allow for a comparison.